Mercury is a potent neurotoxin of significant ecological and public health concern. As wildlife and human exposure occurs largely through fish, 44 states have issued fish consumption advisories regarding Hg contamination including 21 statewide advisories for freshwater and 12 for coastal waters. More than 90% of fish and shellfish consumed by humans come from marine systems but research on Hg fate in marine ecosystems remains limited. Humans and wildlife also have elevated exposures to Hg via fish from reservoirs that are considered Hg hotspots. Project 7 studies mechanisms driving site-to site variation in bioavailability, bioaccumulation, and trophic transfer of Hg in aquatic food webs across a gradient from forested to industrialized watersheds. In both field and experimental studies we will investigate parallel processes influencing inorganic and MeHg fate in reservoirs and estuaries with a focus on lower trophic levels where Hg enters the food web. We predict that site-to-site variation in Hg bioavailability and bioaccumulation will be based on environmental properties such as physical (surface area, watershed area), chemical (pH, nutrients, organic carbon), ecological (benthic and pelagic feeding), and land use factors (agricultural, forested, industrial). We will use field survey and experimental approaches to identify the mechanisms controlling Hg bioavailability to pelagic and benthic food webs and evaluate their relative importance as conduits of Hg trophic transfer. This proposal has three specific aims. Aim 1 characterizes Hg bioavailability, bioaccumulation, and trophic transfer in the field and tests whether the bioaccumulation and transfer of Hg is enhanced in contaminated sites while at the same time diminished by increasing organic matter in the environment. Aim 2 investigates the strength and consistency of benthic vs. pelagic food webs as conduits of Hg to fish and the degree to which the quantity and quality of organic matter in sediments and water diminish Hg uptake and transfer. Aim 3 investigates the role of specific mechanisms such as somatic growth dilution in controlling bioaccumulation of Hg by individual organisms. Results from all three aims will be synthesized into a mathematical contaminant fate model that will assess the net effect of the multiple ecological and biogeochemical factors influencing Hg bioavailability, bioaccumulation, and trophic transfer.